Enable -fvisibility=hidden on Linux.

This will prevent further symbol clashes, and makes the shared libraries
 10 to 20 percent smaller.

We should enable this on other platforms, too, if we can guarantee they'll
 use gcc 4.0 or later and the platform supports it.

Makefile

@@ -304,7 +304,7 @@ ifeq ($(PLATFORM),linux)
   endif
 
   SHLIBEXT=so
-  SHLIBCFLAGS=-fPIC
+  SHLIBCFLAGS=-fPIC -fvisibility=hidden
   SHLIBLDFLAGS=-shared $(LDFLAGS)
 
   THREAD_LIBS=-lpthread